U.S. Pat. No. 6,035,873 issued to the same inventors of this application disclosed a multiple-fold umbrella having gradationally convex telescopic shaft consisting of five or four tubes telescopically engageable with one another. However, there is a large sliding contact area between an outer tube and a corresponding or neighboring inner tube engageable with the outer tube, thereby influencing a smooth folding on unfolding operation of the central shaft when closing or opening the umbrella due to frictional contact between the outer and inner tubes.
The present inventor has found the drawback of the prior patent and invented the present umbrella shaft for a more smooth folding and unfolding operation.
The object of the present invention is to provide a multiple-fold umbrella shaft, consisting of a plurality of tubes telescopically engageable with one another, including an inner tube having concave quadrants designated as Vi, and an outer tube slidably engageable with the inner tube and having concave quadrants designated as Vo, forming a relationship as defined by a formula of Vixe2x88x92Voxe2x89xa71 in order for preventing twisting and vibration of the tubes and for enhancing a smooth folding or unfolding operation when closing or opening the umbrella.